A valve driving apparatus which drives an intake valve and an exhaust valve by using electromagnetic force in an internal combustion engine is already known, as disclosed in Japanese Laid-Open Patent Application No. 9-256825. This type of the valve driving apparatus includes a valve which functions as an intake valve or an exhaust valve, an armature coupled with the intake valve or an exhaust valve, two valve springs which generate force exerted on the intake valve or the exhaust valve, and two electromagnets (an upper electromagnet and a lower electromagnet) disposed in the moving direction of the armature.
In the aforementioned valve driving apparatus, the intake valve or the exhaust valve moves toward the upper electromagnet by the electromagnetic force applied to the armature when an exciting electric current is supplied to the upper electromagnet, because the valve is coupled with the armature. Thereafter, the valve moves toward the lower electromagnet by the force exerted by the valve spring because the electromagnetic force disappears when the exciting current to the upper electromagnet stops. When the exciting current is supplied to the lower electromagnet at the point when the valve reaches near the lower electromagnet, the valve furthermore moves toward the lower electromagnet by the electromagnetic force exerted to the armature. According to the above-mentioned valve driving apparatus, the valve can be driven to open or close, by supplying the exciting current alternately to two of the electromagnets in the appropriate timing.
In order to enhance a volume efficiency of intake air to a combustion chamber of an internal combustion engine, the opening port from an intake port to the combustion chamber may have a large diameter. If the opening port has a large diameter, however, the diameter of the intake valve becomes larger. It results in that the mass of the intake valve is greater. In this case, a moving speed of the intake valve becomes lower. Consequently, the reciprocating interval from a full open position to a full closed position of the intake valve becomes longer. On the other hand, concerning a moving speed of the valve, the greater a spring constant of the valve spring which exerts a force to the valve is, the faster the valve moves. This means that it is better for the spring constant of the intake valve spring to be greater, in order to shorten the reciprocating interval if the diameter of the intake valve is large.
When the spring constant of the intake valve spring becomes higher in the internal combustion engine, however, the exerted spring force on the intake valve becomes greater. It is necessary to increase the electromagnetic force for compensating the excessive spring force, so that the intake valve is held at the full open position or full closed position against the large exerted spring force. Consequently, if the spring constant of the intake valve spring is high, the exciting current necessary for holding the intake valve at the full open position or the full closed position is higher, and it results in the increase of consumed electric power of the intake valve. Therefore, it has an advantage that the spring constant of the intake spring exerting on the intake valve is lower, in order to restrain the consumed electric power for driving the intake valve.
As mentioned above, it is necessary that the spring constant of the intake spring exerted on the intake valve is appropriately determined by taking into consideration reducing the reciprocating interval of the intake valve and reducing the consumed power energy necessary for holding the intake valve at the full open or full closed position.
In the process of opening the intake valve (called intake stroke), the combustion chamber is maintained at the low pressure. In this condition, the intake valve can be opened by a low electromagnetic force, because the pressure which exerts a force toward the intake valve in the closing direction is low.
On the other hand, in the process of opening the exhaust valve (called exhaust stroke), the combustion chamber is at the high pressure, because high pressure combustion gas remains in the combustion chamber after the exhaust stroke. In this case, an amplitude damping value of the exhaust valve becomes higher in the process of the exhaust valve in the opening direction. Greater electromagnetic force is necessary for opening the exhaust valve in the condition where the amplitude damping value of the exhaust valve is higher. Accordingly, it is necessary that a higher exciting electric current is supplied to the lower electromagnet in this case than in the case of opening the intake valve. Then, the consumed electric power for the exhaust valve increases.
The higher the spring constant of the spring exerting the force on the intake or exhaust valve, the lower the above-mentioned amplitude damping value is. If the amplitude damping value is low, it is not necessary to generate a large electromagnetic force in order to move the exhaust valve in the opening direction. Consequently, it is better to adopt the higher spring constant of the spring exerting the force on the exhaust valve, in order to restrain the consumed electric power lower to move the exhaust valve in the opening direction.
In the conventional valve driving apparatus, however, the spring constants of the intake and exhaust springs are set to be equal. Therefore, when the spring constant of the intake spring is designed to gain the optimum characteristics, the consumed electric power in the process of opening the exhaust valve increases because the amplitude damping value is high in the opening process of the exhaust valve. Furthermore, when the spring constant of the exhaust spring is designed to be higher in order to restrain the consumed electric power of the exhaust valve lower, the consumed electric power for holding the intake valve at the full open or full closed position becomes higher, according to the conventional valve driving apparatus.